Multipurpose medical electrode

ABSTRACT

A hydrogel matrix is provided for a multipupose medical electrode having an electrically conductive backing. The matrix comprises a dispersion of a natural or a synthetic hydrophilic polymer, e.g., polyacrylamide, polyacrylic acid, modified starch, or gum karaya, gum acacia, locust bean gum or the like, water, and a humectant such as triethylene glycol or glycerol. The matrix is a flexible sheet having a tacky lower surface for establishing a bond to provide electrical contact with the skin of a patient. The matrix has a resistivity greater than the resistivity of the electrical conductive backing and includes a gelation inhibitor such as a salt of a weak acid, e.g., Mg(OAc) 2  uniformly distributed within the matrix in an amount sufficient to retard gelation of the matrix prior to forming the matrix into a flexible sheet.

This is a continuation of application Set. No. 07/344,698 filed 04/28/1989, now U.S. Pat. No. 5,205,297, which was a continuation-in-part of application Ser. No. 173,589 filed Mar. 25, 1988, now abandoned.

FIELD OF THE INVENTION

The present invention relates to medical electrodes and more particularly to an externally applied multipurpose electrode.

BACKGROUND OF THE INVENTION

External cardiac pacing utilizes relatively high and therefore potentially painful DC current (10-100 mA). This is required to achieve effective stimulation of cardiac muscle located deep within the chest. To deal with these and other problems, various external stimulating electrodes have been previously proposed for medical purposes. For example, U.S. Pat. No. 4,349,030 describes an electrode used for external cardiac pacing which includes as a skin contacting electrically conducting member a porous sponge formed from cellulose in which is absorbed an electrically conductive fluid. Electrical conductivity of the fluid provides a uniform current density to reduce stimulation of local sensory nerves. For example, a 1/4 inch thick cellulose sponge or a piece of gauze is dampened prior to use with tap water or a weak electrolyte. A fluid-like gel can also be used to saturate the sponge or gauze. In order to be absorbed into the fibrous material the gel must have fluid consistency. The sponge is connected to an electrically conductive metal backing having an insulated wire permanently attached to it. In commercial equipment utilizing the patented structure, a male plug is connected to the free end of the insulated wire. During use, the male plug connects to a female receptor located in the external cardiac pacing device or power supply. Other commercially available external cardiac pacing electrodes used in conjunction with specified external cardiac electrical pacing devices are also manufactured with permanent wiring having a wire for supplying current with a unique male plug at its free end that is adapted to connect to a complementary female receptor located in the external pacing equipment.

These prior electrodes suffer from several important shortcomings. First, they can only be used with specific electrical pacing equipment because the male plugs wired in place permanently will only fit a specific piece of equipment. Second, the construction of the electrode makes it relatively expensive because of its size and bulk. Moreover, the electrolyte applied to the sponge must necessarily be in a fluid state to flow into the pores of the sponge or gauze supporting it. Its fluidity however allows the electrolyte to smear, run or leave residue on the patient's body, as well as drying out on the shelf.

U.S. Pat. No. 4,274,420 describes a monitoring and stimulation electrode. The broad statement is made that the resistivity can be anything less than 10,000 ohms-meter (i.e. less than 1,000,000 ohms-centimeter), but the preferred resistivity is less than 1,000 ohms-meter or 100,000 ohms-centimeter. While the patent does disclose a broad range for the purpose of showing that the resistivity is not unduly restrictive, it does not instruct or direct one to a useful level of resistivity employed in accordance with the present invention. Throughout the patent, it is emphasized that there is an advantage in using low AC resistivity in electrodes, e.g. column 3, lines 20-26; column 4, lines 31-32; and example 20, lines 15-20. This is consistent with teachings elsewhere in the patent which suggest the use of relatively large quantities of strongly ionized electrolytes such as sodium chloride. Thus, although the suggestion is made that the electrode can be used for stimulation purposes, there is no disclosure of how one can achieve a painless heart stimulation using constant current, i.e. DC current.

Other stimulating electrodes used for Transcutaneous Electrical Nerve Stimulation (TENS) and neuromuscular stimulation (NMS) utilize a conductive adhesive and a conductive rubber backing. TENS typically employs AC electricity at about 10 mAmp current levels. Because of the alternating nature of this current it generally is considered less painful and irritating than comparable DC levels. One design application of these electrodes, NMS, typically employs DC electricity at about 10 mAmp current levels. This is more similar to cardiac pacing than TENS yet operates at much lower current levels and therefore has much less potential for pain generation than cardiac pacing. Ordinary electrodes, however, do not provide optimal results for painless cardiac stimulation, in part because the electrically conductive matrix will allow current to concentrate in areas of lower impedance occurring in the skin surface. This can occur progressively as the patient is stimulated, causing a runaway condition in which the skin of the patient may even break down electrically, allowing a substantial surge of current to flow through a localized area, causing extreme pain to the patient. Moreover, although conventional TENS and NMS electrodes may provide comfortable stimulation in their designed purpose, both of these electrodes are single purpose electrodes unsuited for cardiac pacing applications due to their relatively high conductivity.

    ______________________________________                                         Typical Applied Current Levels                                                 for Various Stimulation Applications                                           ______________________________________                                         Transcutaneous Nerve Stimulation                                                                       1-10 mA (AC)                                           External Cardiac Pacing                                                                               10-60 mA (DC)                                           Neuro-Muscular Stimulation (NMS)                                                                       1-40 mA (DC)                                           ______________________________________                                    

In view of these and other deficiencies of the prior art, it is a general object of the invention to provide a multipurpose medical electrode suited for Transcutaneous Electrical Nerve Stimulation, neuromuscular stimulation and for cardiac pacing with a provision for reliably preventing current build-up in localized areas and a runaway condition in which a surge of current passes through a localized area of the skin where electrical resistance is broken down under certain applied current loads. A further object is to provide an electrode wherein the electrically conductive skin-contacting matrix is a stable, semi-solid gel that will not run, smear, migrate or flow out onto the patient's body, requires no porous cellulose structure for support, is substantially homogeneous throughout, has a tacky surface adapted to make excellent electrical contact with the skin and to conform to surface irregularities and to flex with the movement of the body and can be applied as a coating layer onto sheet material.

These and other more detailed and specific objects of the invention will become apparent in view of the following specification and drawings which illustrate by way of example but a few of the various ways in which the present invention can be accomplished within the scope of the appended claims.

SUMMARY OF THE INVENTION

The invention provides a multipurpose medical stimulating electrode for external use in contact with the skin of a patient. The electrode is useful for cardiac pacing, TENS, neuromuscular stimulation (NMS) and other transcutaneous electrical stimulation uses. The electrode comprises an electrically conductive flexible backing having a pin socket in conductive relationship therewith. The pin socket includes a pin receptor opening for establishing electrical contact between the electrode and a flexible conductor having a metal pin terminal constructed and arranged to slide into the pin receptor opening. Attached to the backing is a semi-solid hydrogel matrix. The matrix is coated upon the backing and includes an exposed surface adapated to directly contact the skin of the patient. The matrix includes water and a natural or synthetic hydrophilic gum hydrated with water and a humectant as well as a minor amount of an ionizable gelation inhibitor in an amount sufficient to reduce the viscosity of the matrix during coating to prevent premature gelation thereof prior to application of the coating onto the backing. The ionizable gelation inhibitor has a secondary function as an electrolyte for controlling the conductivity of the matrix so as to provide a resistivity between about 1,000 and 10,000 ohm centimeters and thereby preventing current concentration and a consequent runaway condition through specific localized portions of the matrix. This avoids localized areas of high current flow through the skin. In a preferred form of the invention an adhesive layer is provided around the periphery of the electrode to help bond the electrode to the skin and to seal the hydrogel matrix from the environment on all sides.

The invention will now be described by reference to the accompanying figures which illustrate by way of example a preferred form of the invention which illustrates but one of the various ways in which the invention can be accomplished within the scope of the appended claims.

THE FIGURES

FIG. 1 is a perspective view of one preferred form of the invention;

FIG. 2 is a partial plan view taken on line 2--2 of FIG. 1;

FIG. 3 is an enlarged cross-sectional view taken on line 3--3 of FIG. 2;

FIG. 4 is an enlarged vertical cross-sectional view taken on line 4--4 of FIG. 2;

FIG. 5 is a perspective view of the electrode as seen from the underside, i.e., the skin contacting surface with the protective slip sheet or cover sheet removed;

FIG. 6 is a vertical cross-sectional view of the electrode;

FIG. 7 is a perspective view of a pair of electrodes with insulated connecting wires and terminals; and

FIG. 8 is a graph showing the effect of Mg(OAc)₂ on viscosity of the matrix.

DETAILED DESCRIPTION OF THE INVENTION

Refer now to the figures and particularly to FIGS. 1-6 which illustrate an electrode 10 embodying the invention that in this case is circular and includes a protective nonconductive layer 12 which can be formed preferably from a sheet of plastic foam such as foamed polyurethane, polyethylene, foam rubber, etc. The protective layer 12 includes a centrally located upwardly extending tent-like projection 12a having parallel cuts 12b and 12c at each end. The tent-like projection 12a extends over and covers an upwardly extending pin socket 16 that is raisedabove but integral with the upper surface of an electrically conductive backing layer 14. The backing layer 14 can be formed from any suitable material such as metal foil or other electrically conductive plastic, e.g.carbon-containing silicon rubber or electrically conductive doped polyacetylene film. In this case the backing 14 comprises silicone rubber which has been rendered electrically conductive by the addition of carbon particles.

The pin socket 16 is provided with a centrally located horizontally extending aperture 18 which serves as a pin receptor opening for a male contact or terminal to be described below. While the dimensions are not critical, the pin receptor opening has a diameter of about 0.08 inches. The protective foam layer 12 is secured to the conductive layer 14 by means of a suitable adhesive (not shown) and also bonded to a concentric ring 24 of an adhesive supporting material such as a ring of plastic foam,e.g. polyethylene foam in this case of 0.064 inches in thickness that has apressure-sensitive adhesive coating 26 applied to its exposed surface as shown in FIG. 5.

Applied to the lower surface of the conductive layer 14 is an electrically conductive hydrogel matrix composed of a hydrated synthetic or natural hydrocolloid such as a hydrated polyacrylamide or polysaccharide gum or a mixture thereof, together with a liquid hydrating agent, e.g. a polyhydricalcohol and water. The electrically conductive matrix 22 has an exposed skin-contacting surface 22b which is tacky and adapted to form good electrical contact with the skin. It is also sufficiently pliant to conform to the shape of the body contours and to flex with the body. The matrix moreover is in the form of a stable self-supporting or non-fluid gel which although flexible will not flow appreciably during storage or after being applied to the patient's skin.

The pressure-sensitive adhesive layer 26 completely surrounds the peripheryof the electrically conductive matrix 22 and is coextensive with the edge 12d of the protective layer 12. During storage and prior to use the pressure-sensitive layer 26 and the matrix 22 are covered with a removableslip sheet or liner 28 (FIG. 6) which is peeled off just prior to use for the purpose of exposing the pressure-sensitive adhesive 26 and the sticky exposed surface 22b of the hydrogel matrix layer 22. Thus, it can be seen that the electrically conductive backing layer 14 is positioned concentrically within the pressure-sensitive adhesive 26 and foam layer 24so that the adhesive layer extends around the periphery of the matrix 22 toadhere the electrode more reliably to the skin and also to seal the matrix 22 from the environment on all sides.

The composition of the matrix 22 will now be described.

The matrix 22 is in this instance composed of a uniformly electrically conductive flexible hydrated hydrocolloidal mass capable of establishing electrical contact with the skin. It also possesses adhesive properties. The matrix or substrate 22 attached to the conductive layer 14 is substantially homogeneous. The matrix 22 includes a dispersed phase of about 10% to 40% of the total weight of the matrix formed from a hydrophilic synthetic polymer and/or a hydrophilic natural or synthetic gum. The matrix may include synthetic polymers such as polyacrylamide, polyacrylic acid and its salts, polymaleic anhydride, polyvinyl pyrrolidone and its salts, or a modified starch such as pregelatinized starch. Of the naturally occurring gums which may be used are gum Karaya, gum acacia, locust bean gum and other polysaccharide gums. Synthetically modified gums such as modified guar gums and celluloses are also suitable.The synthetic polymers and/or synthetic or natural gums and other polysaccharides constitute the solid hydrated dispersed phase of the matrix. A liquid phase includes a humectant comprising a polyhydric alcohol such as glycerin or triethylene glycol or a mixture thereof and water which comprises about 10% to 70% of the matrix. The polyhydric alcohol may be present in an amount between 10% to 50% of the matrix and the water may be present in an amount of approximately 1.0% to 70% by weight of the matrix. All quantities and percentages stated herein are expressed as weight percent of the total matrix.

The thickness of the finished matrix is typically from approximately 1/32 to 1/4 of an inch. When applied to the skin, body moisture as well as bodysalts and heat are absorbed increasing its tackiness. The bonding and elastic properties of the electrode are enhanced as it ages in contact with the skin.

The polyhydric alcohol swells the hydrophilic polymer and it is involved inhydrogen bonding and cross linking of the polymer. If too much is added thematrix is soft and mushy in nature. In contrast if too little is added the matrix is hard and dry. Triethylene glycol is a less effective hydrating substance than glycerin.

The amount of hydrophilic polysaccharide gum and/or hydrophilic synthetic polymers can be varied from 10% to 40% by weight of the matrix. This material when added builds viscosity and elasticity of the mixture. Excessive amounts cause the formulations to become too elastic or tough tobe coated evenly. If too little is used the matrix may be too soft or runny.

Water is added in an amount of 10% to 70% by weight of the matrix for the dual purpose of controlling resistivity of the skin it is in contact with and swelling the hydrophilic polymers, thus increasing viscoelasticity. Too much water will make the products too soft, but greater amounts will reduce the resistivity of the skin it is in contact with. If not enough water is present in the formulation, the product may be hard and dry and thus inappropriate because it does not properly hydrate the skin.

The previously mentioned components are combined in the following manner. Triethylene glycol is first mixed with the hydrophilic polymer to assure that the polymer is wetted and the mixture is homogeneous. This mixture isthen combined with the most polar liquids such as glycerin and water. This process is preferably completed by means of continuous mixing rather than by batch mixing, since the viscosity of the mixture builds up rapidly withrespect to time of mixing. The liquid components are preferably chilled, e.g. to between about -25° C. and +20° C. This decreases therate of viscosity increase.

If desired, a viscosity stabilizer such as Mg(OAc)₂ can be added to the mixture for decreasing the rate at which viscosity increases in the formulation and for adjusting the electrical conductivity of the matrix. After mixing until homogeneous, the mixture is coated onto a backing, and is then cured by heating it briefly, e.g. to about 100° F. The Mg(OAc)₂ serves a dual function. First, it serves as an inhibitor to prevent gelation of the matrix prior to application to a supporting sheet.It also functions as an electrolyte for modifying the conductivity of the matrix. When used alone, i.e. without other electrolytes, the Mg(OAc)₂ should be present in an amount from about 2% to about 12% byweight and preferably in an amount from about 6% to 9% by weight. Increasing the amount of Mg(OAc)₂ decreases the viscosity building rate of the hydrogel matrix during the mixing procedure just prior to the application of the coating to the backing. Too great an amount will undulyinhibit viscosity increase and too small an amount will allow the gel to thicken too quickly. Moreover, if too great an amount is used the electrical conductivity of the hydrogel matrix will become excessive, producing undesired results. Refer to FIG. 8.

It has been found that the Mg(OAc)₂, when it is present in the formulation, cooperates with the chilling of the mix to increase the time available for coating the matrix. Greater amounts of Mg(OAc)₂ will increase the time available for coating the formulation but too much will unnecessarily increase the conductivity of the matrix which will provide agreater chance for excessive transverse current flow into localized areas of skin. The gel preferably has a minimum of electrolyte and a maximum amount of water present. It has been found by removing electrolytes other than the Mg(OAc)₂ and especially by avoiding salts of a strong acid or base, i.e. a strong electrolyte, that the coating of the matrix onto a backing is facilitated while at the same time avoiding excessive conductivity. While the functionality of the Mg(OAc)₂ is not completely understood, it is known that in solution the salt does not disassociate completely. It therefore appears that the MgOAc⁻ ion acts initially as a cross-linking inhibitor by bonding with the suitable groups present of the natural or synthetic polymer molecules so as to initially prevent cross-linking between them.

The matrix preferably has a relatively high impedance to prevent localized current concentration in a few areas into the skin. The amount of Mg(OAc)₂ and/or other electrolyte present controls the impedance of the matrix. It is preferred that the resistivity of the matrix be maintained between about 1,000 and 10,000 ohm centimeters and preferably about 2,000 to 6,000 ohm centimeters. By maintaining the impedance at thislevel it is possible to employ the electrode for TENS, for various skeletalmuscle stimulation applications and for cardiac pacing as well as preventing localized concentration of current at specific points on the skin surface which is undesirable because of the pain experienced by the patient.

Viscosity buildup is also controlled by the mixing temperature. It is preferred that when triethylene glycol and glycerin are used as humectantsthat they be chilled to about -25° C. to about +10° C. and mixed into the formulation. Typically the mix is chilled to about -5° C. to about +10° C. At this temperature, about one minute is allowed to coat the matrix onto a backing. The matrix should enter the coater at a temperature of about -25° C. to 20° C.and preferably from about -15° C. to 5° C.

Refer now to FIG. 7 which illustrates connectors used in conjunction with the invention. In this case two electrodes 10 and 11, both of which embodythe invention, are used together. One electrode 10 serves as a stimulation electrode and the other electrode 11 serves as a grounding electrode and is adhered to the back. As shown in the figure an insulated electrical connector in the form of an insulated wire 30 is provided at opposite endswith identical pin-style male terminals or connectors 30a and 30b which typically have a diameter of about 0.080 inches. When the connector 30 is used, the terminal 30b is inserted into the opening 18 of the pin socket 16 and the terminal 30a is inserted into a female receptor of test equipment 31 sized to receive the pin 30a. The conductor 30 is a typical cable to interface the electrode 30 with a transcutaneous electrical stimulator for the heart or a neuromuscular stimulator. The stimulator provides a constant current, i.e. DC stimulation of say 50 volts at 50 mAmp. The electrode of the present invention thus provides a resistivity of between about 1,000 and 10,000 ohm centimeters under constant current conditions with a preferred range of about 2,000 to 6,000 ohm centimeters.

In FIG. 7 is also shown an electrical conductor 32 in the form of an insulated electric wire having a 0.080 inch diameter male pin connector 32b at one end and a female snap connector 32a at the opposite end that isconnected during use to an electrical stimulator having a male snap connector 33 adapted to be received into the female receptor 32a. The malepin 32b is the same as 30b so that it fits the opening 18 of the electrode 10.

An electrical connector 34 comprising an insulated electric wire can also be provided. It has a male pinstyle terminal 34c at one end and a male lug-style connector 34b at the other end which in this case projects from a plastic backing plate 34a. The lug 34b is adapted to fit into a socket 35 of another electrical stimulator.

An additional connector 36 is also shown. It includes a pair of electrical conductors in the form of insulated wires 36a and 36b having male pin-style connectors 36c and 36d at one end each adapted to project into an aperture 18. A connector 38 at the opposite end encloses a pair of parallel connector pins 38a and 38b which are connected respectively to the wires 36a and 36b and are adapted to form a connection with still another style of electrical stimulator. The connector 36 is a typical connector for wiring the electrode 10 to an external cardiac pacer. The pins 36c and 36d function to interface with the molded pin socket receptorof the electrode 10.

The backing 14 is relatively highly conductive compared with the matrix 22.The backing 14 when formed from a plastic resin or from rubber rendered electrically conductive, preferably has a resistivity of about 1 to 1,000 ohms/square and preferably 50 to 200, e.g. 100 ohms/square. Typical matrixformulas are shown in the following examples.

EXAMPLE 1

    ______________________________________                                                     Optimum Weight Range                                                           as a % by weight                                                               of the Matrix   Typical                                            ______________________________________                                         Polyacrylamide                                                                               10-40             10%                                            Triethylene Glycol                                                                           10-50             15%                                            Glycerin      10-50             12.5%                                          Water         10-70             60%                                            Mg(OAc).sub.2  1-12             2.5%                                           ______________________________________                                    

EXAMPLE 2

    ______________________________________                                                    Optimum Weight Range                                                           as a % by weight                                                               of the Matrix Typical                                               ______________________________________                                         Polyacrylamide                                                                              10-40           12%                                               Glycerin     10-50           30%                                               Water        10-70           55%                                               Mg(OAc).sub.2                                                                                1-12            3%                                               ______________________________________                                    

EXAMPLE 3

    ______________________________________                                                     Optimum Weight Range                                                           as a % by weight                                                               of the Matrix   Typical                                            ______________________________________                                         Polyacrylamide                                                                               10-40             15%                                            Triethylene Glycol                                                                           10-50             20%                                            Glycerin      10-50             15%                                            Water         10-70             50%                                            ______________________________________                                    

EXAMPLE 4

    ______________________________________                                                     Optimum Weight Range                                                           as a % by weight                                                               of the Matrix   Typical                                            ______________________________________                                         Polyacrylamide                                                                               10-30             10%                                            Karaya         5-15              5%                                            Triethylene Glycol                                                                           10-50             15%                                            Glycerin      10-50             12.5%                                          Citric Acid   1-5               1.5%                                           SnCl.sub.2    .1-2              1.0%                                           Water          1-70             60%                                            ______________________________________                                    

EXAMPLE 5

    ______________________________________                                                     Optimum Weight Range                                                           as a % by weight                                                               of the Matrix   Typical                                            ______________________________________                                         Polyacrylamide                                                                               10-40              5%                                            Karaya         5-15             10%                                            Triethylene Glycol                                                                           10-40             15%                                            Glycerin      10-40             15%                                            Water          1-70             55%                                            ______________________________________                                    

Many variations of the invention within the scope of the appended claims will be apparent to those skilled in the art once the principles describedabove are understood. 

What is claimed is:
 1. A hydrogel matrix for a multipurpose medical electrode having an electrically conductive backing, said matrix comprising:a dispersion of a natural or synthetic hydrophilic polymer, water and a humectant, said matrix is a flexible sheet having a tacky lower surface for establishing a bond to provide electrical contact with the skin of a patient, said electrically conductive backing having a predetermined resistivity of a first value, said matrix having a resistivity of a second value that is greater than said first value, and a gelation inhibitor uniformly distributed within the matrix in an amount sufficient to retard gelation of the matrix prior to forming the matrix into said flexible sheet.
 2. The matrix of claim 1 wherein the gelation inhibitor comprises a salt of a weak acid.
 3. The matrix of claim 2 wherein the salt is Mg(OAc)₂.
 4. The matrix of claim 1 wherein the polymer is at least one of the following: polyacrylamide or karaya, and the humectant is at least one of the following: triethylene glycol or glycerine.
 5. The matrix of claim 1 wherein a pressure-sensitive sheet is provided at the periphery of said matrix in a position to surround the matrix for bonding the electrode to the skin and to seal the matrix from the environment.
 6. The matrix of claim 1 wherein the matrix has a resistivity between about 2,000 and 10,000 ohm-centimeters for reducing current concentration in localized areas of the matrix, for preventing a runaway condition in which a disproportionate amount of current passes through a localized area of the skin of a patient so as to inhibit the perception of pain by the patient.
 7. A biomedical electrode device for external use in contact with the skin of a patient comprising:a flexible sheet a matrix comprising a flexible layer of a hydrogel connected to the flexible sheet and having a surface to be exposed for direct contact with the skin of the patient, to conform to the body contours and to establish electrical contact therewith, said hydrogel matrix comprises a natural or synthetic hydrophilic polymer hydrated with water and a humectant, a minor amount of an ionizable gelation inhibitor is present in an amount sufficient to reduce the viscosity of the hydrogel matrix so as to prevent premature gelation thereof during formation of said hydrogel matrix layer, and the resistivity of the matrix is sufficient to prevent current buildup through specific localized portions of the matrix and thereby avoid a runaway condition of high current flow in localized areas of the skin so as to inhibit the sensation of pain by the patient.
 8. The electrode device of claim 7 wherein the gelation inhibitor is a salt of a weak acid.
 9. The electrode device of claim 8 wherein the gelation inhibitor is a magnesium salt of a weak acid.
 10. The electrode device of claim 9 wherein the magnesium salt is Mg(OAc)₂.
 11. The electrode device of claim 7 wherein a pressure-sensitive adhesive layer is provided to extend around the periphery of said matrix to bond the electrode to the skin and to seal the hydrogel matrix from the atmosphere.
 12. The electrode device of claim 7 wherein the hydrophilic polymer comprises at least one member selected from the group consisting of polyacrylamide and karaya.
 13. The electrode device of claim 7 wherein said humectant comprises at least one member selected from the group consisting of triethylene glycol and glycerin.
 14. The electrode device of claim 7 wherein the matrix includes a synthetic hydrocolloid comprising polyacrylamide in an amount by weight of between about 10% to about 40% and a liquid phase comprising water and at least one humectant, said liquid phase being present in an amount by weight of about 20% to about 88%, and the gelation inhibitor is Mg(OAc)₂ in the amount by weight of about 1% to about 12%.
 15. The electrode device of claim 14 wherein the hydrogel matrix has a resistivity between about 2,000 and 10,000 ohm-centimeters to produce said reduced current concentration in a localized area. 